1. Field of the Invention
The present invention relates to a process for the transmission of data between subscribers of a public dial telephone system and, more particularly, to a process for use with a system including: a telephone, a data modem having a tone transmitter and a tone receiver and a data device, all connected to the dial telephone system.
2. Description of the Prior Art
Switching from telephoning to data transmission by a data modem may be carried out either automatically or manually, by operation of a data key. Before switching to the data transmission mode, it must be determined for each subscriber whether the subscriber is the calling or the called party, as described in German Patent 25 02 681, equivalent to GB PS 1,494,717.
A process for making such a determination is prescribed in the CCITT Standards presently in effect. Modems suitable for use in a public dial telephone system are described, for example in CCITT Standards V.21 for duplex operation and V.23 for half duplex operation. CCITT Standard V.25 prescribes the transmission of a data answering tone by a called subscriber prior to data transmission. This tone serves, on the one hand, to cancel echo suppressors on the transmission path and on the other hand, reception of the data answering tone informs a calling subscriber that the called subscriber is equipped with a data modem and that a data exchange with this subscriber is therefore possible. The calling subscriber, or the subscriber's data modem, must also be able to detect the end of the data answering tone, because only then can the transmission of data be started. According to CCITT, the data answering tone should last for 3.3.+-.0.7 seconds and should have a frequency of 2100 Hz.
The data modems for both the calling and called subscribers must be capable of detecting, before receiving the data transmission, whether `its` subscriber is being called, (called state) or is itself the calling party (calling state), so that, as the called party, it can effect the transmission of the data answering tone. For duplex operation, a knowledge of the called state or calling state is also required for selection of a proper channel position, because, according to CCITT, the data of the calling subscriber are, in principle, transmitted to the called subscriber via Channel 1, whereas Channel 2 serves for transmission in the opposite direction. CCITT Standard V.25 describes the procedure for establishment of the appropriate data connection in the case of an automatic call and of automatic call answer. However, no detailed statements concerning manual operation are made in the CCITT Standards. The subscribers of two manual stations are advised to make arrangements with each other for the initiation of data transmission.
German Patent 12 75 093 described a process for automatic channel selection for 2-channel or duplex data modems. For each subscriber, a channel switching relay is assigned to these data modems. In the case of the calling subscriber, only the subscriber's own dial pulses are selected as a criterion for selection of the transmission channel, while only the arriving ringing characters are used in the case of the called subscriber. The switching for channel assignment is effected by a flip-flop.
In the process according to German Patent 25 02 681 mentioned above, to differentiate between the calling state and the called state, a circuit with different timers is used, which timers analyze whether the data answering tone arrived within a time that is adjustable on one of the timers or not. The timer is started with the connection of the data modem to the telephone line. Thus, precise timing standards must be adhered to.